On Tue, Jun 11, 2019 at 01:57:16PM +0200, Thomas Zimmermann wrote:
> Acquiring drm_client_dev.modeset_mutex after the locks in drm_fb_helper.dev
> creates a deadlock with drm_setup_crtcs() as shown below:
>
> [ 4.959319] fbcon: radeondrmfb (fb0) is primary device
> [ 4.993952] Console: switching to colour frame buffer device 240x67
> [ 4.994040]
> [ 4.994041] ======================================================
> [ 4.994041] WARNING: possible circular locking dependency detected
> [ 4.994042] 5.2.0-rc4-1-default+ #39 Tainted: G E
> [ 4.994043] ------------------------------------------------------
> [ 4.994043] systemd-udevd/369 is trying to acquire lock:
> [ 4.994044] 00000000fb622acb (&client->modeset_mutex){+.+.}, at: drm_fb_helper_pan_display+0x103/0x1f0 [drm_kms_helper]
> [ 4.994055]
> [ 4.994055] but task is already holding lock:
> [ 4.994055] 0000000028767ae4 (crtc_ww_class_mutex){+.+.}, at: drm_modeset_lock+0x42/0xf0 [drm]
> [ 4.994072]
> [ 4.994072] which lock already depends on the new lock.
> [ 4.994072]
> [ 4.994072]
> [ 4.994072] the existing dependency chain (in reverse order) is:
> [ 4.994073]
> [ 4.994073] -> #3 (crtc_ww_class_mutex){+.+.}:
> [ 4.994076] lock_acquire+0x9e/0x170
> [ 4.994079] __ww_mutex_lock.constprop.18+0x97/0xf40
> [ 4.994080] ww_mutex_lock+0x30/0x90
> [ 4.994091] drm_modeset_lock+0x42/0xf0 [drm]
> [ 4.994102] drm_modeset_lock_all_ctx+0x1f/0xe0 [drm]
> [ 4.994113] drm_modeset_lock_all+0x5e/0x1a0 [drm]
> [ 4.994163] intel_modeset_init+0x60b/0xda0 [i915]
> ..
> [ 4.994253]
> [ 4.994253] -> #2 (crtc_ww_class_acquire){+.+.}:
> [ 4.994255] lock_acquire+0x9e/0x170
> [ 4.994270] drm_modeset_acquire_init+0xcc/0x100 [drm]
> [ 4.994280] drm_modeset_lock_all+0x44/0x1a0 [drm]
> [ 4.994320] intel_modeset_init+0x60b/0xda0 [i915]
> ..
> [ 4.994403]
> [ 4.994403] -> #1 (&dev->mode_config.mutex){+.+.}:
> [ 4.994405] lock_acquire+0x9e/0x170
> [ 4.994408] __mutex_lock+0x62/0x8c0
> [ 4.994413] drm_setup_crtcs+0x17c/0xc50 [drm_kms_helper]
> [ 4.994418] __drm_fb_helper_initial_config_and_unlock+0x34/0x530 [drm_kms_helper]
> [ 4.994450] radeon_fbdev_init+0x110/0x130 [radeon]
> ..
> [ 4.994535]
> [ 4.994535] -> #0 (&client->modeset_mutex){+.+.}:
> [ 4.994537] __lock_acquire+0xa85/0xe90
> [ 4.994538] lock_acquire+0x9e/0x170
> [ 4.994540] __mutex_lock+0x62/0x8c0
> [ 4.994545] drm_fb_helper_pan_display+0x103/0x1f0 [drm_kms_helper]
> [ 4.994547] fb_pan_display+0x92/0x120
> [ 4.994549] bit_update_start+0x1a/0x40
> [ 4.994550] fbcon_switch+0x392/0x580
> [ 4.994552] redraw_screen+0x12c/0x220
> [ 4.994553] do_bind_con_driver.cold.30+0xe1/0x10d
> [ 4.994554] do_take_over_console+0x113/0x190
> [ 4.994555] do_fbcon_takeover+0x58/0xb0
> [ 4.994557] notifier_call_chain+0x47/0x70
> [ 4.994558] blocking_notifier_call_chain+0x44/0x60
> [ 4.994559] register_framebuffer+0x231/0x310
> [ 4.994564] __drm_fb_helper_initial_config_and_unlock+0x2fd/0x530 [drm_kms_helper]
> [ 4.994590] radeon_fbdev_init+0x110/0x130 [radeon]
> ..
>
> This problem was introduced in
>
> d81294afe drm/fb-helper: Remove drm_fb_helper_crtc
>
> Reversing the lock ordering in pan_display_legacy() fixes the issue. The fix
> was suggested by Daniel Vetter.
>
> Signed-off-by: Thomas Zimmermann <tzimmermann@xxxxxxx>
> Fixes: d81294afeecdacc8d84804ba0bcb3d39e64d0f27
I think for ocd consistency it be nice to pull the lock out from both
pan_display_atomic and pan_disaply_legacy and move it into
drm_fb_helper_pan_display. Like we do drm_fb_helper_dpms or
drm_fb_helper_setcmap or restore_fbdev_mode_force.
Either way Reviewed-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
> ---
> drivers/gpu/drm/drm_fb_helper.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
> index 7b388674a456..d6991f07cb17 100644
> --- a/drivers/gpu/drm/drm_fb_helper.c
> +++ b/drivers/gpu/drm/drm_fb_helper.c
> @@ -1586,8 +1586,8 @@ static int pan_display_legacy(struct fb_var_screeninfo *var,
> struct drm_mode_set *modeset;
> int ret = 0;
>
> - drm_modeset_lock_all(fb_helper->dev);
> mutex_lock(&client->modeset_mutex);
> + drm_modeset_lock_all(fb_helper->dev);
> drm_client_for_each_modeset(modeset, client) {
> modeset->x = var->xoffset;
> modeset->y = var->yoffset;
> @@ -1600,8 +1600,8 @@ static int pan_display_legacy(struct fb_var_screeninfo *var,
> }
> }
> }
> - mutex_unlock(&client->modeset_mutex);
> drm_modeset_unlock_all(fb_helper->dev);
> + mutex_unlock(&client->modeset_mutex);
>
> return ret;
> }
> --
> 2.21.0
>
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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